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  #41  
Old 09-11-2018, 08:46 AM
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For darks and bias I would do them for each photo session separately (for specific ISO of course)- they will be dependent on envirimment teperature - especially darks.
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  #42  
Old 09-11-2018, 08:51 AM
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Merlin66 (Ken)
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Jethro,
I've entered your data into CCDCalc.
(Based on the Sony SLT having 3.88micron pixel and an array of 6056 x 4020)
You can see the wide field coverage!

Hope this helps. (You can change the target object in CCDCalc to see the differences)
You can then use the FOV (789.5 x 524.5 arcmin) to produce an overlay for your planetarium program (I use CdC) to arrange your target.....
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  #43  
Old 09-11-2018, 03:00 PM
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Quote:
Originally Posted by bojan View Post
JA, total number of photons delivered to the sensor (as a whole) is irrelevant for exposure time etc... what is relevant is photon flux per pixel (or flux per mm^2) and that will be the same for the same lens (including f/ number).


ONLY difference with different sensor size will be the field of view.
Well I think I kind of got that , since I emphasised for the sake of completeness virtually exactly that earlier, when I said:

Quote:
Originally Posted by JA View Post
...
It should be noted however: that the light per unit area (of sensor), and hence the exposure, on both sensors will/can be the same, BUT the Total light delivered to the sensor, if you could count the number of photons delivered to the sensor, will NOT be.
...

Best
JA

Last edited by JA; 09-11-2018 at 03:20 PM.
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  #44  
Old 09-11-2018, 03:54 PM
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Well, then we should really, finally have that beer, without much delay
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  #45  
Old 09-11-2018, 04:05 PM
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Well, then we should really, finally have that beer, without much delay
Good Idea Bojan!
Cheers .....

Best
JA
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  #46  
Old 11-11-2018, 07:16 PM
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Darks need the information of the camera as it was during the actual photo shoot. Info like heat and dead pixels.
So you take them at the end of each session with all the settings the same as the actual pictures, ISO, exposure time, Aperture.

Flat frames are used to eliminate vignetting and dust from the stacked light frames. So in your current setup, almost no moving parts, it's cool to take them any time you want and re-use the resulting masterflat from DSS for many months. (For each individual lens a separate masterflat.)
I don't know anything about bias. Never bothered. But that is not a qualified attitude!

I am thoroughly impressed with your cropped Orion photo! Is that a stack of several 3.5 secs exposures or a single frame?

Get that barndoor mount going!
Imagine how much more detail you get to capture and investigate from Orion with several exposures of, say, 15 seconds or more!!
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  #47  
Old 12-11-2018, 07:40 AM
Jethro777 (Jethro)
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Thanks for the feedback,

The moon is kind of out now, so I'm waiting until the new moon for the next set.

Perhaps by then the rest of the parts will arrive for the barn-door tracker and I will see what I can gather then. I'm also going to test an excellent terrestrial lens, and see if it has any astro characteristics.

Silv, I am attaching some 100% RAWs as shot so you can see processed vs. unprocessed and different times. As you can see, a 3.2 second shot is just too much. As Bojan and JA pointed out, the 500 / 100mm rule is wrong because of the ASPC sensor - I'm still getting drift even at 3.2.

The 2 second shots were okay, taken in good night conditions (dark site), but the Maia nebula wasn't visible.

The RAW 100.jpg is the 3.2 second (f/2.5, 3200ISO) exposure from which I got LDN 1641. You can see the 2 (f/2.8 1600ISO), and 2.5 second (f/4.5, 1600 ISO) exposures.

Mind you - most shots are a total exposure of just 1.5 minutes max.
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  #48  
Old 12-11-2018, 08:27 AM
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Jethro,

Regardless of sensor size, you will be getting drift after couple of seconds of exposure with this particular focal length.
It is simply the consequence of Earth rotation, 15 per hour (or 15 arcsec per second), at celestial equator.
That means, with 100mm FL lens, (and M42 is close to equator) you will have drift ~4.85um per second (which is close to your pixel sze, 3.88um).
With longer lens, that drift will be proportionally longer, and it's value does not depend on the sensor size, it depends only on FL of the lens.


With 2.5 sec exposure, you have drift of 12 arcsec, or ~3.12 pixels on your photo (see below),
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  #49  
Old 12-11-2018, 08:49 AM
Jethro777 (Jethro)
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Quote:
Originally Posted by bojan View Post
Jethro,

Regardless of sensor size, you will be getting drift after couple of seconds of exposure with this particular focal length.
It is simply the consequence of Earth rotation, 15 per hour (or 15 arcsec per second), at celestial equator.
That means, with 100mm FL lens, (and M42 is close to equator) you will have drift ~4.85um per second (which is close to your pixel sze, 3.88um).
With longer lens, that drift will be proportionally longer, and it's value does not depend on the sensor size, it depends only on FL of the lens.


With 2.5 sec exposure, you have drift of 12 arcsec, or ~3.12 pixels on your photo (see below),
So, really from that, is there "optimal drift" until I get the tracker going?
I think I have been wrongly trying to "eliminate it" when really, I will always have it given the current Setup. I have been seeing drift and thinking "exposure time too long".

Switching to a 35mm lens again would allow longer exposure but less resolution, what would be better for dss stacking / final quality?
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  #50  
Old 12-11-2018, 08:59 AM
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Quote:
Originally Posted by Jethro777 View Post
So, really from that, is there "optimal drift" until I get the tracker going?
I think I have been wrongly trying to "eliminate it" when really, I will always have it given the current Setup. I have been seeing drift and thinking "exposure time too long".

Switching to a 35mm lens again would allow longer exposure but less resolution, what would be better for dss stacking / final quality?

Switching back to 35mm and you can have the ~3x longer exposure for the same drift...

I think you just have to build that barn door tracker
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  #51  
Old 12-11-2018, 09:25 AM
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Also, bear in mind M42 belongs in "worst case" cathegory in terms of drift...

So objects away from celestial equator (closer to SCP) will have lower drift, and you can consequently use longer exposures.
Another thing, without equatorial tracking, you will also notice "field rotation"

All that discussion won't be necessary once you have equatorial mount and tracking (including here barn door tracker).

Last edited by bojan; 12-11-2018 at 09:35 AM.
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  #52  
Old 12-11-2018, 09:57 AM
Jethro777 (Jethro)
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Update on the tracking project:

Boards have been organised.
They are chip board however so I have tried to weatherproof them.
Normally, the thread on a tripod is 3"8, however the tripod I have secured doesn't have that width, it uses the M8 width, so I have secured that rod and various bolts I am anticipating I will need.

I am concerned about how to secure the bottom board onto the tripod securely. It will need to hold the board, the dslr and the lens of course.

I am using a template to build it, so it should work.

https://barn-door-tracker.co.uk

I can't quite see how it is secured.

Perhaps this will be one clearer when I secure the final parts.
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  #53  
Old 12-11-2018, 10:23 AM
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Have you considered driving this with motor?
I think this tripod is not sturdy enough, it will move as you are turning the knob.... resulting in images worse that you already have.
This thread will have to move with very little friction.

Some sort of 1 rpm motor (powered from mains) may be a quantum leap from this design.
Or his one (requires transformer 240V -> 12V AC)

(This particular motor from ebay may be designed for 60Hz mains.. so it may be running 20% slower in AU, requiring 20% shorter arm of the barn-door tracker).
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  #54  
Old 12-11-2018, 10:58 AM
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Well, this is exciting.

Will it hold? It is an old school 80s tripod, quite heavy and solid. I think it will carry the weight if I can secure it properly.
I am not sure about mounting the camera at that length, I might shorten the length of rod connecting it to the tracker.

I intend to lubricate the rod well with easy glide (a dry lubricant, PTFE) not wd40 which will make it very smooth and resist debris/dust.

Now the motor is interesting. What resources should I read on how to add gears? I am not a math or engineer head sadly...

Picture of planned tracker for those that missed the pic earlier in the thread.
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  #55  
Old 12-11-2018, 11:29 AM
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Yes, by all means shorthen the rod - perhaps the camera should be mounted directly on the board.
I would also have the knob (motor) on the bottom side..


Those motors are supposed to be 1rpm (1 rotation per minute) so the shaft should be directly mounted (or via 1:1 gear reduction) to the threaded rod.
Perhaps with something like this.


As mentioned earlier, you can use 60Hz motor for this, but the distance between hinge and thredaed rod should be 20% smaller (or thread pich should be finer)

Last edited by bojan; 12-11-2018 at 11:50 AM.
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  #56  
Old 12-11-2018, 12:36 PM
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Just thinking, (and I feel really silly), I need the whole tripod to align the hinge/side of the wood. That means I actually require a ball head or similar to mount the DSLR.
Another part to acquire... Any ideas where I can get one cheap?

This is needed regardless of whether the DSLR is mounted on the rod or the board itself.
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  #57  
Old 12-11-2018, 12:38 PM
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I think I might have my old tracker around the house somewhere. I'll have a look for it and send some pics. I used a plastic gear from a hobby store with the nut that suits your threaded rod glued to it.

Then used a cheap stepper motor with a spur gear to drive the gear attached to the threaded rod. Used an arduino to control the stepper motor. The wiring and coding was quite simple. The stepper motor and arduino can be purchased for like $5 each on Ebay. while the plastic gears were maybe a few dollars each.

I think the extra effort to motorise it will be well worth it. As Bojan mentioned unless your tripod is particularly sturdy your movements to spin the threaded rod will be transmitted through to the camera. (It takes a very small amount of movement to cause trailing in an image.)

I'll get some pics of my old tracker up tonight so you can see the design.
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  #58  
Old 12-11-2018, 12:41 PM
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Quote:
Originally Posted by Jethro777 View Post
Just thinking, (and I feel really silly), I need the whole tripod to align the hinge/side of the wood. That means I actually require a ball head or similar to mount the DSLR.
Another part to acquire... Any ideas where I can get one cheap?

This is needed regardless of whether the DSLR is mounted on the rod or the board itself.
You could make a wedge that is cut at the same angle as the latitude of your imaging location. (Ball head for polar aligning can be painful as you have to get both directions aligned simultaneously).

This will get you in the ball park of your altitude adjustment. Any finer adjustment can be done through changing the height of the front tripod leg.
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  #59  
Old 12-11-2018, 12:52 PM
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Or, you could mount the base directly on tripod head, which should have elevation axis (I think this is how it was done on UK website)...


As Aaron mentioned, arduino is an excellent way to do those things, but you have to be a little bit techical...

It is also good because you will be independent from mains supply.
I was using those motors and drivers for many things (focus, tracking etc.).

Last edited by bojan; 12-11-2018 at 01:06 PM.
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  #60  
Old 12-11-2018, 12:58 PM
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Quote:
Originally Posted by Jasp05 View Post
You could make a wedge that is cut at the same angle as the latitude of your imaging location. (Ball head for polar aligning can be painful as you have to get both directions aligned simultaneously).

This will get you in the ball park of your altitude adjustment. Any finer adjustment can be done through changing the height of the front tripod leg.
Right. So, you just move the tripod left or right to align from location and adjust the legs a touch for "up" and "down". Can confirm I am going to try a green laser to align it.

So, I can try and use the existing head I have to move the DSLR.

Should I go curved rod, or straight rod? Curved looks correct right now and let me stay aligned longer, right?
I think I will make the attempt to motorize it.
1rpm, 3rpm or something else? Why this speed?
I need to decide on the motor (9v/even 6v compatible sounds good for portability?)
Will a lm317 kit be needed to regulate the speed? Will that work with 6v?

Perhaps this motor would be suitable? https://www.ebay.com.au/itm/DC-6V-1r...AAAOSwH05ZsLCx

Also, this will probably become helpful: https://blarg.co.uk/astronomy/barn-d...ker-calculator (thanks Aaron)

Last edited by Jethro777; 12-11-2018 at 06:11 PM.
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